Generally, in preparing liquid crystal display devices, liquid crystal cells comprising liquid crystals and polarizing plates are basically required and suitable adhesive layers or pressure-sensitive adhesive layers have to be used for binding them. In addition, for improving functions of liquid crystal display devices, a phase retardation plate, a compensation plate for wide view angle, a brightness enhancement film, and the like may be used, with additionally adhered to the polarizing plate.
Major structure forming a liquid crystal display device comprises, generally, a uniformly aligned liquid crystal layer; a polarizing plate with a multi-layer structure, incorporated into a pressure-sensitive adhesive layer or an adhesive layer, based on a liquid crystal cell consisted of a clear glass plate or plastic sheet material containing an electrode layer; a phase retardation plate; and an additional functional film layer and the like.
The structure of polarizing plate is one comprising an iodine compound or a dichroic polarizing material aligned in a certain direction. To protect these polarizing elements, multi-layers are formed on both sides using a protective film such as triacetyl cellulose (TAC). In addition, the polarizing plate may additionally comprise a phase retardation film, or a compensation film for wide view angle such as a liquid crystal type film, in a shape having a unidirectional molecular alignment.
The aforementioned films are made of materials having different molecular structures and compositions, and so have different physical properties. Especially, under a high temperature and/or humidity condition, the dimensional stability according to shrinkage or expansion of materials having a unidirectional molecular alignment is insufficient. As a result, if the polarizing plate is fixed by a pressure-sensitive adhesive and shrunk or expanded under a high temperature and/or humidity condition, then stress is concentrated on the TAC layer, birefringence is developed and thereby a light leakage phenomenon occurs. In this case, a negative birefringence is usually caused.
Meanwhile, the pressure-sensitive adhesive layer needs high cohesion strength at high temperature to maintain the durability, for which partially cross-linked viscoelastic materials are used. When the partially cross-linked structure is introduced into the pressure-sensitive layer, the pressure-sensitive layer has the residual stress under the given stress and the polymer in the cross-linked structure is aligned in the specific direction to develop birefringence. Under such alignment, general acrylic pressure-sensitive adhesives develop negative birefringence.
Recently, monitor size of computers, and the like becomes larger, and the demand is rapidly increased for LCD TVs using polarizing plates. As such panels become larger, polarizing plates also become larger, and thereby the residual stresses of the TAC layer and the pressure-sensitive adhesive layer becomes higher and the magnitude of negative birefringence is extremely increased.
Among methods of minimizing light leakage under such residual stresses, a method adjusting zero birefringence under the residual stress is considered, by means of adding (blending) materials representing positive birefringence in the final pressure-sensitive adhesive layer under strain-induced stress or copolymerizing some special acrylic monomers, which shows positive birefringence under strain-induced stress.
KR laid-open patent publication No. 2003-0069461 discloses a method of compensating negative birefringence of acrylic pressure-sensitive adhesive layer under strain-induced stress by incorporating 0.01 to 40 parts by weight of low molecular weight organic molecules having a positive value of birefringence under the strain-induced residual stress into the acrylic pressure-sensitive adhesive layer. In such case, the modulus of pressure-sensitive adhesive may be lowered, due to use of the low molecular weight organic molecules incorporated into the pressure-sensitive adhesive layer and thus the tailoring property may be lowered on processing the polarizing plate. The low molecular weight organic molecules is more likely moved into the interface for long-term storage or caused the phase separation with the acrylic pressure-sensitive adhesive. In addition, Mc (molecular weight between two cross-linking points) in the cross-linking structure is substantially increased, depending on the plasticizing effect due to the low molecular weight organic molecules. There is a problem that a large quantity of low molecular weight organic molecules should be introduced, since the increased Mc reduces the residual stress and the alignment of low molecular weight organic molecules.